Machine Structural Steel Excellent in Machinability and Strength Properties

ABSTRACT

The invention provides a machine structural steel excellent in machinability and strength properties that has good machinability over a broad range of machining speeds and also has high impact properties and high yield ratio, which machine structural steel comprises, in mass %, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, total Al: greater than 0.05% and not greater than 0.3%, Sb: less than 0.0150% (including 0%), and total N: 0.0035 to 0.020%, solute N being limited to 0.0020% or less, and a balance of Fe and unavoidable impurities.

FIELD OF THE INVENTION

This invention relates to a machine structural steel that is to bemachined and particularly to a machine structural steel excellent inmachinability and strength properties that is amenable to machining overa broad spectrum of machining speeds ranging from relatively low-speedmachining with a high-speed steel drill to relatively high speedmachining such as longitudinal turning with a super-steel coated tool.

DESCRIPTION OF THE RELATED ART

Although recent years have seen the development of steels of higherstrength, there has concurrently emerged a problem of decliningmachinability. An increasing need is therefore felt for the developmentof steels that maintain excellent strength without experiencing adecline in machining performance. Addition of machinability-enhancingelements such as S, Pb and Bi is known to be effective for improvingsteel machinability. However, while Pb and Bi are known to improvemachinability and to have relatively little effect on forgeability, theyare also known to degrade strength properties.

Moreover, Pb is being used in smaller quantities these days owing to thetendency to avoid use because of concern about the load Pb puts on thenatural environment. S improves machinability by forming inclusions,such as MnS, that soften in a machining environment, but MnS grains arelarger than the those of Pb and the like, so that it readily becomes astress concentration raiser. Of particular note is that at the time ofelongation by forging or rolling, MnS produces anisotropy, which makesthe steel extremely weak in a particular direction. It also becomesnecessary to take such anisotropy into account during steel design. WhenS is added, therefore, it becomes necessary to utilize a technique forreducing the anisotropy.

As pointed out in the foregoing, it has been difficult to achieve goodstrength properties and good machinability simultaneously becauseaddition of machinability-enhancing elements degrades the strengthproperties. Further technological innovation is therefore needed toenable simultaneous realization of satisfactory steel machinability andstrength properties.

This situation has led to efforts to provide a machine structural steelenabling prolongation of machine tool life by, for example,incorporating a total of 0.005 mass % or greater of at least one memberselected from among solute V, solute Nb and solute Al, and furtherincorporating 0.001% or greater of solute N, thereby enabling nitridesformed by machining heat during machining to adhere to the tool tofunction as a tool protective coating (see Japanese Patent Publication(A) No. 2004-107787). In addition, there has been proposed a machinestructural steel that achieve improved shavings disposal and mechanicalproperties by defining C, Si, Mn, S and Mg contents, defining the ratioof Mg content to S content, and optimizing the aspect ratio and numberof sulfide inclusions in the steel (see Japanese Patent No. 3706560).The machine structural steel taught by Patent No. 3706560 defines thecontent of Mg as 0.02% or less (not including 0%) and the content of Al,when included, as 0.1% or less.

SUMMARY OF THE INVENTION

However, the foregoing existing technologies have the followingdrawbacks. The steel taught by Japanese Patent Publication (A) No.2004-107787 is liable not to give rise to the aforesaid phenomenonunless the amount of heat produced by the machining exceeds a certainlevel. The machining speed must therefore be somewhat high to realizethe desired effect, so the invention has a problem in the point that theeffect cannot be anticipated in the low speed range. Japanese Patent No.3706560 is totally silent regarding the strength properties of the steelit teaches. Moreover, the steel of this patent is incapable of achievingadequate strength properties because it gives no consideration tomachine tool life or yield ratio.

The present invention was achieved in light of the foregoing problemsand has as its object to provide a machine structural steel that hasgood machinability over a broad range of machining speeds and also hashigh impact properties and high yield ratio.

The machine structural steel excellent in machinability and strengthproperties according to the present invention comprises, in mass %, C,0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S:0.001 to 0.15%, total Al: greater than 0.05% and not greater than 0.3%,Sb: less than 0.0150% (including 0%), and total N, 0.0035 to 0.020%,solute N being limited to 0.0020% or less, and a balance of Fe andunavoidable impurities.

The machine structural steel can further comprise, in mass %, Ca: 0.0003to 0.0015%.

The machine structural steel can further comprise, in mass %, one ormore elements selected from the group consisting of Ti: 0.001 to 0.1%,Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to 1.0%.

The machine structural steel can further comprise, in mass %, one ormore elements selected from the group consisting of Mg: 0.0001 to0.0040%, Zr: 0.0003 to 0.01%, and REMs: 0.0001 to 0.015%.

The machine structural steel can further comprise, in mass %, one ormore elements selected from the group consisting of Sn: 0.005 to 2.0%,Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005to 0.5%, and Pb: 0.005 to 0.5%.

The machine structural steel can further comprise, in mass %, one or twoelements selected from the group consisting of Cr: 0.01 to 2.0% and Mo:0.01 to 1.0%.

The machine structural steel can further comprise, in mass %, one or twoelements selected from the group consisting of Ni: 0.05 to 2.0% and Cu:0.01 to 2.0%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a region from which a Charpy impact testpiece was cut.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are explained in detailin the following. The machine structural steel excellent inmachinability and strength properties according to the present inventionachieves the foregoing object by providing a machine structural steelwherein solute N acting to degrade machinability and impact propertiesis minimized by adjusting the amounts of added N and nitride-formingelements such as Al, wherein effective cutting performance isestablished with respect to a broad cutting speed range extending fromlow to high speed by ensuring presence of suitable amounts of solute Alserving to improve high-temperature embrittlement property andmachinability, and Sb serving to produce a matrix embrittlement effect,and forming a crystal structure exhibiting high-temperatureembrittlement effect and cleavage, thereby ensuring an appropriateamount of AlN serving to improve machinability, and wherein high impactproperties are also realized by increasing Al addition so that at theslab stage segregation is made smaller and MnS of highly uniformdispersibility (type III MnS by SIMS analysis) is made more abundantthan in conventional Al-killed steel. Moreover, the steel furtherachieves a high yield ratio owing to fine precipitation of AlN andpresence of solute Al.

Specifically, the machine structural steel according to the presentinvention comprises, in mass %, C, 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn:0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, total Al: greaterthan 0.05% and not greater than 0.3%, Sb: less than 0.0150% (including0%), and total N, 0.0035 to 0.020%, solute N being limited to 0.0020% orless, and a balance of Fe and unavoidable impurities.

The individual elements constituting the machine structural steel of thepresent invention and the contents thereof of will first be explained.In the ensuing explanation, percentage composition by mass of the steelcomponents is denoted simply by the symbol %. C, 0.1 to 0.85%

C has a major effect on the fundamental strength of the steel. When theC content is less than 0.1%, adequate strength cannot be achieved, sothat large amounts of other alloying elements must be incorporated. WhenC content exceeds 0.85%, machinability declines markedly because carbonconcentration becomes nearly hypereutectoid to produce heavyprecipitation of hard carbides. In order to achieve sufficient strength,the present invention therefore defines C content as 0.1 to 0.85%. Thepreferred lower limit of C content is 0.2%. Si: 0.01 to 1.5%

Si is generally added as a deoxidizing element but also contributes toferrite strengthening and temper-softening resistance. When Si contentis less than 0.01%, the deoxidizing effect is insufficient. On the otherhand, an Si content in excess of 1.5% degrades the steel's embrittlementand other properties and also impairs machinability. Si content istherefore defined as 0.01 to 1.5%. The preferred upper limit of Sicontent is 1.0%.

Mn: 0.05 to 2.0%

Mn is required for its ability to fix and disperse sulfur (S) in thesteel in the form of MnS and also, by dissolving into the matrix, toimprove hardenability and ensure good strength after quenching. When Mncontent is less than 0.05%, the steel is embrittled because S thereincombines with Fe to form FeS. When Mn content is high, specifically whenit exceeds 2.0%, base metal hardness increases to degrade coldworkability, while its strength and hardenability improving effectssaturates. Mn content is therefore defined as 0.05 to 2.0%.

P: 0.005 to 0.2%

P has a favorable effect on machinability but the effect is not obtainedat a P content of less than 0.005%. When P content is high, specificallywhen it exceeds 0.2%, base metal hardness increases to degrade not onlycold workability but also hot workability and casting properties. Pcontent is therefore defined as 0.005 to 0.2%,

S: 0.001 to 0.15%

S combines with Mn to produce MnS that is present in the steel in theform of inclusions. MnS improves machinability but S must be added to acontent of 0.001% or greater for achieving this effect to a substantialdegree. When S content exceeds 0.15%, the impact value of the steeldeclines markedly. In the case of adding S to improve machinability,therefore, the S content is made 0.001 to 0.15%.

Total Al: Greater than 0.05% and not Greater than 0.3%

Al not only forms oxides but also promotes precipitation of AlN, whichcontributes to grain size control and machinability, and furtherimproves machinability by passing into solid solution. Al must be addedto a content of greater than 0.05% in order to form solute Al in anamount sufficient to enhance machinability. Al also affects the form ofMnS grains/precipitation. Moreover, when Al is added in an amountexceeding 0.05%, segregation at the slab stage can be made smaller andMnS of highly uniform dispersibility (type III MnS by SIMS analysis) bemade more abundant than in a conventional Al-killed steel. This makes itpossible to obtain a machine structural steel also having high impactproperties and further to achieve a high yield ratio owing to fineprecipitation of AlN and the presence of solute Al. However,machinability starts to decline when total Al content exceeds 0.3%.Total Al content is therefore defined as greater than 0.05% and notgreater than 0.3%. The lower limit of total Al content is preferably0.08% and more preferably 0.1%.

Sb: Less than 0.0150% (Including 0%)

Sb improves machinability by suitably embrittling ferrite. This effectof Sb is pronounced particularly when solute Al content is high but isnot observed when Sb content is less than 0.0005%. When Sb content ishigh, specifically when it reaches 0.0150% or greater, Sbmacro-segregation becomes excessive, so that the impact value of thesteel declines markedly. Sb content is therefore defined as 0.0005% orgreater and less than 0.0150%. When high machinability is not requiredor total Al is greater than 0.1%, addition of Sb can be omitted (Sbcontent of 0%).

Total N, 0.0035 to 0.020%

N, which is present not only as solute N but also in nitrides of Ti, AlV and the like, suppresses austenite grain growth. However, nosubstantial effect is obtained at a total N content of less than0.0035%. When total N content exceeds 0.020%, it leads to the occurrenceof roll marks during rolling. Total N content is therefore defined as0.0035 to 0.020%.

Solute N, 0.0020% or Less

Solute N hardens the steel. Of particular concern is that it shortenscutting tool life by causing steel near the cutting edge to harden underdynamic strain aging. It also causes occurrence of roll marks duringrolling. High solute N content, specifically a content in excess of0.0020%, aggravates tool wear during cutting because cutting resistancerises due to increased local hardness. Solute N content is thereforeheld to 0.0020% or less. This helps to reduce tool wear. Moreover, highsolute N content also degrades impact properties by causing matrixembrittlement, but such matrix embrittlement can also be mitigated byholding solute N content to 0.0020% or less. Solute N content as termedhere means the value obtained by subtracting the N content of AlN, NbN,TiN, VN and other such nitrides from total N content. It can becalculated, for example, in accordance with Equation (1) shown below,using the total N content determined by the inert gas fusion thermalconductivity method and the N content of nitrides determined by SPEED(Selective Potentiostatic Etching by Electrolytic Dissolution) analysisand indophenol absorbency analysis of residue electrolytically extractedusing a 0.1 μm filter.

(Solute N content)=(Total N content)−(Nitride N content)  (1)

Solute N content can be lowered by the methods explained below:

1) Hold total N content to a low level within the range defined by thepresent invention. Although total N is defined as 0.020% or less, it ispreferably held to 0.01% or less and more preferably to 0.006% or less.2) When total N content is high, it is helpful to increase the amount ofN compounds by adding suitable amounts of Al, a nitride-forming element,as well as other nitride-forming elements.3) Solute N reduction by fine precipitation of nitrides is preferable ina machine structural steel from the viewpoint of inhibiting graincoarsening. Taking into account that reduction of solute N content byfine precipitation of nitrides requires holding at a high temperatureenabling more complete solution treatment into N and nitride-formingelement content, solution heat treatment is conducted at a temperatureof 1100° C. or greater, preferably 1200° C. or greater, and morepreferably 1250° C. or greater, whereafter precipitation is performed byconducting a heat treatment such as normalizing or carburization. Ofparticular note is that in the case of AlN, solute N can be reduced byutilizing prolonged retention near 850° C. to increase precipitation. By“prolonged” here is meant 0.8 hr or greater, preferably 1 hr or greaterand more preferably 1.2 hr or greater.

The machine structural steel of the present invention can contain Ca inaddition to the foregoing components.

Ca: 0.0003 to 0.0015%

Ca is a deoxidizing element that forms oxides in the steel. In themachine structural steel of the present invention, which has a total Alcontent of greater than 0.05% and not greater than 0.3%, Ca formscalcium aluminate (CaOAl₂O₃). As CaOAl₂O₃ is an oxide having a lowermelting point than Al₂O₃, it improves machinability by constituting atool protective film during high-speed cutting However, thismachinability-improving effect is not observed when the Ca content isless than 0.0003%. When Ca content exceeds 0.0015%, CaS forms in thesteel, so that machinability is instead degraded. Therefore, when Ca isadded, its content is defined as 0.0003 to 0.0015%.

When the machine structural steel of the present invention needs to begiven high strength by forming carbides, it can include in addition tothe foregoing components one or more elements selected from the groupconsisting of Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, andV: 0.01 to 1.0%. Ti: 0.001 to 0.1%

Ti forms carbonitrides that inhibit austenite grain growth andcontribute to strengthening. It is used as a grain size control elementfor preventing grain coarsening in steels requiring high strength andsteels requiring low distortion. Ti is also a deoxidizing element thatimproves machinability by forming soft oxides. However, these effects ofTi are not observed at a content of less than 0.001%, and when thecontent exceeds 0.1%, Ti has the contrary effect of degrading mechanicalproperties by causing precipitation of insoluble coarse carbonitridesthat cause hot cracking. Therefore, when Ti is added, its content isdefined as 0.001 to 0.1%.

Nb: 0.005 to 0.2%

Nb also forms carbonitrides. As such, it is an element that contributesto steel strength through secondary precipitation hardening and toaustenite grain growth inhibition and strengthening. Ti is thereforeused as a grain size control element for preventing grain coarsening insteels requiring high strength and steels requiring low distortion.However, no high strength imparting effect is observed at an Nb contentof less than 0.005%, and when Nb is added to a content exceeding 0.2%,it has the contrary effect of degrading mechanical properties by causingprecipitation of insoluble coarse carbonitrides that cause hot cracking.Therefore, when Nb is added, its content is defined as 0.005 to 0.2%.

W: 0.01 to 1.0%

W is also an element that forms carbonitrides and can strengthen thesteel through secondary precipitation hardening. However, no highstrength imparting effect is observed when W content is less than 0.01%,Addition of W in excess of 1.0% has the contrary effect of degradingmechanical properties by causing precipitation of insoluble coarsecarbonitrides that cause hot cracking. Therefore, when W is added, itscontent is defined as 0.01 to 1.0%.

V: 0.01 to 1.0%.

V is also an element that forms carbonitrides and can strengthen thesteel through secondary precipitation hardening. It is suitably added tosteels requiring high strength. However, no high strength impartingeffect is observed when V content is less than 0.01%, Addition of V inexcess of 1.0% has the contrary effect of degrading mechanicalproperties by causing precipitation of insoluble coarse carbonitridesthat cause hot cracking. Therefore, when V is added, its content isdefined as 0.01 to 1.0%.

When the machine structural steel of the present invention is subjectedto deoxidization control for controlling sulfide morphology, it cancomprise in addition to the foregoing components one or more elementsselected from the group consisting of Mg: 0.0001 to 0.0040%, Zr: 0.0003to 0.01%, and REMs: 0.0001 to 0.015%.

Mg: 0.0001 to 0.0040%

Mg is a deoxidizing element that forms oxides in the steel. When Aldeoxidization is adopted, Mg reforms Al₂O₃, which impairs machinability,into relatively soft and finely dispersed MgO and Al₂O₃—Mg. Moreover,its oxide readily acts as a precipitation nucleus of MnS and thus worksto finely disperse MnS. However, these effects are not observed at an Mgcontent of less than 0.0001%. Moreover, while Mg acts to make MnSspherical by forming a metal-sulfide complex therewith, excessive Mgaddition, specifically addition to a content of greater than 0.0040%,degrades machinability by promoting simple MgS formation. Therefore,when Mg is added, its content is defined as to 0.0001 to 0.0040%.

Zr: 0.0003 to 0.01%.

Zr is a deoxidizing element that forms an oxide in the steel. The oxideis thought to be ZrO₂, which acts as a precipitation nucleus for MnS.Since addition of Zr therefore increases the number of MnS precipitationsites, it has the effect of uniformly dispersing MnS. Moreover, Zrdissolves into MnS to form a metal-sulfide complex therewith, thusdecreasing MnS deformation, and therefore also works to inhibit MnSgrain elongation during rolling and hot forging. In this manner, Zreffectively reduces anisotropy. But no substantial effect in theserespects is observed at a Zr content of less than 0.0003%. On the otherhand, addition of Zr in excess of 0.01% radically degrades yield.Moreover, by causing formation of large quantities of ZrO₂, ZrS andother hard compounds, it has the contrary effect of degrading mechanicalproperties such as machinability, impact value, fatigue properties andthe like. Therefore, when Zr is added, its content is defined as to0.0003 to 0.01%.

REMs: 0.0001 to 0.015%

REMs (rare earth metals) are deoxidizing elements that formlow-melting-point oxides that help to prevent nozzle clogging duringcasting and also dissolve into or combine with MnS to decrease MnSdeformation, thereby acting to inhibit MnS shape elongation duringrolling and hot forging. REMs thus serve to reduce anisotropy. However,this effect does not appear at an REM content of less than 0.0001%. Whenthe content exceeds 0.015%, machinability is degraded owing to theformation of large amounts of REM sulfides. Therefore, when REMs areadded, their content is defined as 0.0001 to 0.015%.

When the machine structural steel of the present invention is to beimproved in machinability, it can include in addition to the foregoingcomponents one or more elements selected from the group consisting ofSn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003to 0.2%, Bi: 0.005 to 0.5%, and Pb: 0.005 to 0.5%.

Sn: 0.005 to 2.0%

Sn extends tool life by embrittling ferrite and also improves surfaceroughness. These effects are not observed when the Sn content is lessthan 0.005%, and the effects saturate when Sn is added in excess of2.0%. Therefore, when Sn is added, its content is defined as 0.005 to2.0%.

Zn: 0.0005 to 0.5%

Zn extends tool life by embrittling ferrite and also improves surfaceroughness. These effects are not observed when the Zn content is lessthan 0.0005%, and the effects saturate when Zn is added in excess of0.5%. Therefore, when Zn is added, its content is defined as 0.0005 to0.5%.

B: 0.0005 to 0.015%

B, when in solid solution, has a favorable effect on grain boundarystrength and hardenability. When it precipitates, it precipitates as BNand therefore helps to improve machinability. These effects are notnotable at a B content of less than 0.0005%. When B is added to acontent of greater than 0.015%, the effects saturate and mechanicalproperties are to the contrary degraded owing to excessive precipitationof BN. Therefore, when B is added, its content is defined as 0.0005 to0.015%.

Te: 0.0003 to 0.2%

Te improves machinability. It also forms MnTe and, when co-present withMnS, decreases MnS deformation, thereby acting to inhibit MnS shapeelongation. Te is thus an element effective for reducing anisotropy.These effects are not observed when Te content is less than 0.0003%, andwhen the content thereof exceeds 0.2%, the effects saturate and hotrolling ductility declines, increasing the likelihood of flaws.Therefore, when Te is added, its content is defined as: 0.0003 to 0.2%.

Bi: 0.005 to 0.5%

Bi improves machinability. This effect is not observed when Bi contentis less than 0.005%. When it exceeds 0.5%, machinability improvementsaturates and hot rolling ductility declines, increasing the likelihoodof flaws. Therefore, when Bi is added, its content is defined as 0.005to 0.5%.

Pb: 0.005 to 0.5%

Pb improves machinability. This effect is not observed when Pb contentis less than 0.005%. When it exceeds 0.5%, machinability improvementsaturates and hot rolling ductility declines, increasing the likelihoodof flaws. Therefore, when Pb is added, its content is defined as 0.005to 0.5%.

When the machine structural steel of the present invention is to beimparted with strength by improving its hardenability and/ortemper-softening resistance, it can include in addition to the foregoingcomponents one or two elements selected from the group consisting of Cr:0.01 to 2.0% and Mo: 0.01 to 1.0%.

Cr: 0.01 to 2.0%

Cr improves hardenability and also imparts temper-softening resistance.It is therefore added to a steel requiring high strength. These effectsare not obtained at a Cr content of less than 0.01%. When Cr content ishigh, specifically when it exceeds 2.0%, the steel is embrittled owingto formation of Cr carbides. Therefore, when Cr is added, its content isdefined as 0.01 to 2.0%.

Mo: 0.01 to 1.0%

Mo imparts temper-softening resistance and also improves hardenability.It is therefore added to a steel requiring high strength. These effectsare not obtained at an Mo content of less than 0.01%. When Mo is addedin excess of 1.0%, its effects saturate. Therefore, when Mo is added,its content is defined as 0.01 to 1.0%.

When the machine structural steel of the present invention is to besubjected to ferrite strengthening, it can include in addition to theforegoing components one or two elements selected from the groupconsisting of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0%.

Ni: 0.05 to 2.0%

Ni strengthens ferrite, thereby improving ductility, and is alsoeffective for hardenability improvement and anticorrosion improvement.These effects are not observed an Ni content of less than 0.05%. When Niis added in excess of 2.0%, mechanical property improving effectsaturates and machinability is degraded. Therefore, when Ni is added,its content is defined as 0.05 to 2.0%.

Cu: 0.01 to 2.0%

Cu strengthens ferrite and is also effective for hardenabilityimprovement and anticorrosion improvement. These effects are notobserved a Cui content of less than 0.01%. When Cu is added in excess of2.0%, mechanical property improving effect saturates. Therefore, when Cuis added, its content is defined as 0.01 to 2.0%. A particular concernregarding Cu is that its effect of lowering hot rollability may lead tooccurrence of flaws during rolling. Cu is therefore preferably addedsimultaneously with Ni.

As explained in the foregoing, the machine structural steel of thepresent invention is minimized in solute N content and thereforeachieves better machinability and impact properties than conventionalmachine structural steels. Moreover, total Al content and Sb content arecontrolled to suitable levels to ensure presence of proper amounts ofsolute Al, Sb and AlN serving to improve machinability, therebyestablishing effective cutting performance with respect to a broadcutting speed range extending from low to high speed. The steel alsoachieves a high yield ratio owing to fine precipitation of the AlN andpresence of solute Al. In addition, excellent impact properties arerealized by appropriately regulating the contents of elements affectingMnS precipitation so as to obtain an abundance of MnS of highly uniformdispersibility.

The machine structural steel excellent in machinability and strengthproperties according to the present invention can be produced byhot-forging a billet having the aforesaid steel composition into a barat a temperature of 1200° C. or greater, subjecting the bar to solutionheat treatment at a temperature of 1100° C. or greater, and then to aheat treatment such as normalizing or carburization. Of particular noteis that in the case of a steel containing the carbide AlN, a machinestructural steel markedly reduced in solute N can be obtained byprolonged retention following the solution heat treatment at 1100° C. orgreater for 0.8 hr or greater, preferably 1 hr or greater, and morepreferably 1.2 hr or greater.

EXAMPLES First Set of Examples

The effects of the present invention will now be specifically explainedgiving Examples and Comparative Examples. In this set of Examples,steels of the compositions shown in Table 1 and Table 2, 150 kg each,were produced in a vacuum furnace, hot-forged under a temperaturecondition of 1250° C., and elongation-forged into 65-mm diameter bars.The properties of the Example and Comparative Example steels wereevaluated by subjecting them to machinability testing, Charpy impacttesting and tensile testing by the methods set out below. In Table 2,underlining indicates a value outside the invention range.

TABLE 1 Composition (mass %) No. C Si Mn P S Cr V Sb Ca Ttl Al Ttl N SolN Other In- 1 0.42 0.19 0.80 0.014 0.022 — — 0.0100 0.0012 0.110 0.00520.0012 vention 2 0.40 0.25 0.76 0.012 0.034 — — 0.0089 0.0008 0.0510.0060 0.0013 Exam- 3 0.41 0.24 0.76 0.013 0.038 — 0.1 0.0086 — 0.0510.0060 0.0013 ples 4 0.40 0.23 0.78 0.015 0.038 — — 0.0067 — 0.0520.0045 0.0014 5 0.43 0.23 0.75 0.011 0.022 — — 0.0087 — 0.060 0.00490.0012 Mg: 0.0020 6 0.43 0.20 0.77 0.013 0.039 — — 0.0074 — 0.051 0.00650.0013 Ti: 0.04 7 0.44 0.20 0.78 0.012 0.040 — — 0.0068 — 0.052 0.00750.0016 Nb: 0.02 8 0.41 0.21 0.77 0.011 0.047 — — 0.0083 — 0.090 0.00580.0014 W: 0.2 9 0.45 0.22 0.79 0.012 0.045 — — 0.0058 — 0.080 0.00550.0013 Ni: 0.2 10 0.43 0.23 0.71 0.011 0.051 — — 0.0071 — 0.110 0.00450.0017 Cu: 0.5 11 0.44 0.22 0.72 0.014 0.041 — — 0.0087 — 0.053 0.00520.0010 Sn: 0.05 12 0.45 0.20 0.74 0.010 0.033 — — 0.0069 — 0.070 0.00510.0014 Zn: 0.007 13 0.43 0.24 0.76 0.015 0.041 — — 0.0077 — 0.090 0.00530.0019 B: 0.002 14 0.45 0.22 0.71 0.011 0.043 — — 0.0073 — 0.080 0.00460.0015 Te: 0.002 15 0.43 0.19 0.74 0.011 0.051 1.0 — 0.0051 — 0.0900.0047 0.0016 16 0.44 0.21 0.72 0.013 0.023 0.1 — 0.0085 — 0.070 0.00480.0013 17 0.42 0.21 0.73 0.012 0.048 — — 0.0088 — 0.110 0.0071 0.0010Ti: 0.03, Mg: 0.0025 18 0.41 0.20 0.72 0.012 0.035 — — 0.0059 — 0.0900.0075 0.0011 Ti: 0.04, Zn: 0.004 19 0.42 0.24 0.74 0.013 0.040 1.0 —0.0083 — 0.060 0.0071 0.0012 Ti: 0.03 20 0.44 0.23 0.75 0.010 0.034 — —0.0089 — 0.110 0.0077 0.0015 Ti: 0.03, Cu: 0.3 21 0.40 0.20 0.71 0.0100.037 — — 0.0074 — 0.110 0.0054 0.0010 Ti: 0.02, Mg: 0.0025, Sn: 0.04 220.42 0.21 0.73 0.012 0.053 1.1 — 0.0098 — 0.110 0.0074 0.0019 Ti: 0.03,Mg: 0.0025 23 0.43 0.21 0.77 0.014 0.052 — — 0.0071 — 0.070 0.00620.0019 Ti: 0.03, Mg: 0.0025, Cu: 0.4 24 0.41 0.19 0.74 0.013 0.054 1.0 —0.0076 — 0.100 0.0061 0.0015 Ti: 0.02, Mg: 0.0025, Sn: 0.04 25 0.43 0.230.71 0.014 0.021 — — 0.0058 — 0.060 0.0060 0.0012 Ti: 0.03, Mg: 0.0025,Sn: 0.04, Cu: 0.3 26 0.43 0.25 0.76 0.013 0.024 1.0 — 0.0085 — 0.0700.0074 0.0010 Ti: 0.03, Mg: 0.0025, Cu: 0.4 27 0.45 0.23 0.72 0.0150.034 1.0 — 0.0086 — 0.100 0.0055 0.0012 Ti: 0.03, Sn: 0.04 28 0.41 0.190.78 0.011 0.025 — — 0.0087 — 0.080 0.0061 0.0016 Ti: 0.03, Sn: 0.04.Cu: 0.3 29 0.41 0.21 0.70 0.015 0.025 0.9 — 0.0057 — 0.051 0.0062 0.0017Ti: 0.04, Sn: 0.04. Cu: 0.3 30 0.44 0.23 0.71 0.012 0.036 1.0 — 0.0052 —0.060 6.0056 0.0012 Ti: 0.03, Cu: 0.3

TABLE 2 Composition (mass %) No. C Si Mn P S Cr V Sb Ca Ttl Al Ttl N SolN Other Examples 31 0.44 0.20 0.77 0.014 0.055 — — 0.0081 — 0.110 0.00470.0010 Mg: 0.0025, Zn: 0.003 32 0.45 0.21 0.79 0.011 0.029 1.0 — 0.0099— 0.060 0.0049 0.0013 Mg: 0.0019, Zn: 0.003 33 0.43 0.21 0.74 0.0100.038 — — 0.0078 — 0.110 0.0048 0.0014 Mg: 0.0022, Ca: 0.3 34 0.42 0.250.77 0.011 0.036 1.0 — 0.0087 — 0.090 0.0046 0.0014 Mg: 0.0020, Sn: 0.0435 0.44 0.25 0.78 0.015 0.055 — — 0.0079 — 0.100 0.0050 0.0018 Mg:0.0025, Sn: 0.04, Cu: 0.1 36 0.42 0.19 0.74 0.013 0.022 1.0 — 0.0062 —0.052 0.0047 0.0019 Mg: 0.0021, Sn: 0.02, Cu: 0.1 37 0.41 0.19 0.770.010 0.025 1.1 — 0.0050 — 0.110 0.0049 0.0019 Mg: 0.0029, Cu: 0.1 380.43 0.20 0.79 0.011 0.020 1.0 — 0.0060 — 0.060 0.0049 0.0010 Sn: 0.0439 0.42 0.23 0.80 0.015 0.048 — — 0.0086 — 0.070 0.0046 0.0016 Sn: 0.03,Cu: 0.1 40 0.41 0.19 0.78 0.010 0.042 1.0 — 0.0069 — 0.100 0.0046 0.0009Sn: 0.04, Cu: 0.1 41 0.43 0.21 0.79 0.010 0.035 0.9 — 0.0080 — 0.0800.0046 0.0014 Cu: 0.2 42 0.44 0.19 0.77 0.013 0.042 — — 0.0087 — 0.0600.0055 0.0014 Nb: 0.01, Mg: 0.0026, Sn: 0.04, Ca: 0.3 Comparative 430.45 0.24 0.78 0.010 0.025 — — 0.0069 — 0.025 0.0052 0.0018 Examples 440.43 0.25 0.76 0.010 0.041 — — 0.0092 — 0.035 0.0051 0.0019 45 0.41 0.240.73 0.011 0.035 — — 0.0098 — 0.040 0.0053 0.0017 46 0.44 0.25 0.780.014 0.022 — — 0.0059 — 0.030 0.0034 0.0019 47 0.41 0.24 0.72 0.0110.051 — — 0.0087 — 0.003 0.0049 0.0034 48 0.44 0.25 0.77 0.015 0.052 — —0.0062 — 0.358 0.0062 0.0011 49 0.41 0.21 0.72 0.013 0.021 — — 0.0055 —0.103 0.0058 0.0025 50 0.42 0.20 0.73 0.013 0.037 — — 0.0077 — 0.1530.0057 0.0026 51 0.44 0.24 0.79 0.013 0.038 — — 0.0157 — 0.067 0.00540.0016 52 0.45 0.23 0.76 0.010 0.036 — — 0.0175 — 0.103 0.0049 0.0010 530.44 0.19 0.73 0.014 0.044 — — 0.0211 — 0.243 0.0046 0.0016 54 0.45 0.190.71 0.010 0.025 — — 0.0223 — 0.060 0.0046 0.0009

Machinability Test

Machinability testing was conducted with respect to Example andComparative Example steels that had been elongation-forged under heatingat 1250° C. by first subjecting them to heat treatment consisting ofnormalization under temperature condition of 850° C. for 1 hr, 0.5 hr inthe case of Comparative Examples No. 49 and No. 50, followed byair-cooling. A machinability evaluation test piece was then cut fromeach heat-treated steel and the machinabilities of the Example andComparative Example steels were evaluated by conducting drill boringtesting under the cutting conditions shown in Table 3 and tolongitudinal turning testing under the conditions shown in Table 4. Themaximum cutting speed VL1000 enabling cutting up to a cumulative holedepth of 1000 mm was used as the evaluation index in the drill boringtest, and the maximum width VB_max of wear of the relief flank after 10min was used as the evaluation index in the longitudinal turning test.

TABLE 3 Cutting Speed: 10-120 m/min conditions Feed: 0.25 mm/rev Cuttingfluid: Water-soluble cutting oil Drill Drill diameter: 3 mm NACHIordinary drill Overhang: 45 mm Other Hole depth: 9 mm Tool life: Untilbreakage

TABLE 4 Cutting Cutting speed: 250 m/min conditions Feed: 0.3 mm/revDepth of cut: 1.5 mm Dry cutting Tool Holder: PTGNR2525M16 Tool shape:TNMG160408N- UZ Material: AC2000

Charpy Impact Test

FIG. 1 is a diagram showing a region from which a Charpy impact testpiece was cut. In the Charpy impact test, first, as shown in FIG. 1, acylinder 2 measuring 25 mm in diameter was cut from a steel 1heat-treated by the same method and under the same conditions as theaforesaid machinability test piece so that its axis was perpendicular tothe elongation-forging direction of the steel 1. Next, the cylinder 2was held under temperature condition of 850° C. for 1 hr, 0.5 hr in thecase of Comparative Examples No. 49 and No. 50, then oil-quenched bycooling to 60° C., and further subjected to tempering with water coolingin which it was held under temperature condition of 550° C. for 30 min.Next, the cylinder 2 was machined to fabricate a Charpy test piece 3 inconformance with JIS Z 2202, which was subjected to a Charpy impact testat room temperature in accordance with the method prescribed by JIS Z2242. Absorbed energy per unit area (J/cm²) was adopted as theevaluation index.

Tensile Test

A cylinder 2 sampled parallel to the elongation-forging direction wasoil-quenched and tempered by the same methods and under the sameconditions as in the aforesaid Charpy impact test, whereafter it wasprocessed into a tensile test piece measuring 8 mm in parallel sectiondiameter and 30 mm in parallel section length, and then tensile testedat room temperature in accordance with the method prescribed by JIS Z2241. Yield ratio (=(0.2% proof stress YP)/(tensile strength TS) wasadopted as the evaluation index.

The results of the foregoing tests are shown in Tables 5 and 6.

TABLE 5 Impact VL1000 VB_max value No. (m/min) (μm) (J/cm²⁾ YP/TSExamples 1 87 118 27 0.85 2 89 119 22 0.83 3 85 124 20 0.89 4 88 126 250.82 5 85 125 20 0.84 6 89 123 21 0.86 7 89 128 22 0.83 8 92 129 22 0.869 89 126 20 0.86 10 92 122 21 0.82 11 91 121 21 0.83 12 87 130 22 0.8413 90 127 22 0.82 14 90 125 21 0.84 15 90 125 24 0.86 16 85 121 26 0.8717 93 128 24 0.86 18 86 124 22 0.85 19 90 128 25 0.86 20 89 126 20 0.8221 89 121 24 0.84 22 96 125 21 0.82 23 93 129 22 0.83 24 94 126 23 0.8225 82 126 26 0.82 26 86 120 25 0.84 27 89 130 26 0.85 28 86 127 20 0.8629 83 129 20 0.84 30 86 122 24 0.86 31 95 129 26 0.86 32 89 130 22 0.8333 89 123 21 0.87 34 90 126 24 0.85 35 94 121 22 0.82 36 83 127 20 0.8537 83 121 20 0.85 38 82 127 30 0.83 39 93 127 21 0.83 40 90 124 27 0.8641 89 127 23 0.84 42 91 126 21 0.87

TABLE 6 Impact VL1000 VB_max value No. (m/min) (μm) (J/cm²⁾ YP/TSComparative 43 54 149 21 0.68 Examples 44 62 141 24 0.66 45 60 142 250.67 46 53 158 22 0.68 47 64 178  9 0.65 48 48 178 22 0.82 49 62 149 160.83 50 69 150 15 0.85 51 99 122 14 0.85 52 98 124 11 0.87 53 104  12812 0.84 54 100  128 13 0.85

The Steels No. 1 to No. 42 shown in Tables 1, 2 and 5 are Examples ofthe present invention, and the steels No. 43 to No. 51 shown in Tables 2and 6 are Comparative Example steels. As shown in Tables 5 and 6, thesteels of Examples No 1 to No. 42 exhibited good values for all of theevaluation indexes, namely VL1000, VB_max, impact value (absorbedenergy), and YP/TS (yield ratio), but the steels of the ComparativeExamples were each inferior to the Example steels in at least one of theproperties. Specifically, the steels of Comparative Examples No. 43 toNo. 46 had total Al contents below the range of the present inventionand were therefore inferior to the Example steels in machinabilityevaluation index VL1000 and yield ratio (YP/TS). Moreover, the steel ofComparative Example No. 47 had a total Al content far below the range ofthe present invention, so that its solute N content was above the rangeof the present invention, and the steel was therefore inferior to thesteels of the Examples in machinability (VL1000, VB_max), impact value,and yield ratio (YP/TS).

The steel of Comparative Example No. 48 had a total Al content above therange of the present invention, so that its hardness increased, and thesteel was therefore inferior in machinability (VL1000, VB_max). Thesteels of Comparative Examples No. 49 and No. 50 were maintained at 850°C., the temperature at which AlN readily precipitates, for a shorterholding time than the steels of the Examples, so that their solute Ncontents were above the range of the present invention, and the steelswere therefore inferior to the steels of the Examples in machinability(VL1000, VB_max) and impact value. The steels of Comparative ExamplesNo. 51 to No. 54 had Sb contents above the range of the presentinvention and were therefore inferior to the steels of the Examples inimpact value.

Second Set of Examples

In this set of Examples, steels of the compositions shown in Table 7 andTable 8, 150 kg each, were produced in a vacuum furnace, hot-forgedunder a temperature condition of 1250° C., and elongation-forged into 65mm diameter bars. The properties of the Example and Comparative Examplesteels were evaluated by subjecting them to machinability testing,Charpy impact testing and tensile testing by the methods set out below.In Tables 7 and 8, underlining indicates a value outside the inventionrange.

TABLE 7 Composition (mass %) No C Si Mn P S Cr Ca Ttl Al Ttl N Sol NOther Examples 1 0.44 0.25 0.76 0.015 0.017 — 0.0000 0.121 0.0052 0.00122 0.44 0.26 0.76 0.015 0.012 — 0.0006 0.101 0.0052 0.0012 3 0.44 0.250.75 0.016 0.010 — 0.0008 0.250 0.0060 0.0013 4 0.44 0.25 0.76 0.0150.008 — 0.0010 0.075 0.0045 0.0011 5 0.46 0.26 0.76 0.015 0.013 — 0.00060.099 0.0049 0.0012 Mg: 0.0020 6 0.44 0.24 0.74 0.015 0.011 — 0.00080.193 0.0065 0.0013 Ti: 0.04 7 0.45 0.25 0.74 0.015 0.013 — 0.0008 0.1780.0075 0.0016 Nb: 0.02 8 0.44 0.24 0.74 0.015 0.011 — 0.0006 0.1690.0058 0.0014 W: 0.2 9 0.45 0.24 0.74 0.016 0.010 — 0.0012 0.175 0.00550.0013 Ni: 0.2 10 0.46 0.26 0.76 0.015 0.014 — 0.0005 0.142 0.00450.0017 Cu: 0.5 11 0.44 0.26 0.75 0.015 0.015 — 0.0007 0.127 0.00520.0010 Sn: 0.05 12 0.44 0.25 0.76 0.015 0.011 — 0.0004 0.147 0.00510.0014 Zn: 0.007 13 0.45 0.24 0.76 0.014 0.011 — 0.0012 0.144 0.00530.0019 B: 0.002 14 0.45 0.26 0.75 0.015 0.011 — 0.0012 0.187 0.00460.0015 Te: 0.002 15 0.41 0.24 0.78 0.015 0.014 1.0 0.0010 0.108 0.00470.0016 16 0.44 0.25 0.76 0.015 0.013 0.1 0.0012 0.112 0.0048 0.0013 170.44 0.24 0.74 0.015 0.010 — 0.0006 0.131 0.0071 0.0010 Ti: 0.03, Mg:0.0025 18 0.45 0.26 0.75 0.016 0.010 — 0.0009 0.109 0.0075 0.0010 Ti:0.04, Zn: 0.004 19 0.41 0.24 0.75 0.016 0.010 1.0 0.0008 0.168 0.00710.0012 Ti: 0.03 20 0.44 0.26 0.74 0.016 0.010 — 0.0011 0.113 0.00770.0015 Ti: 0.03, Cu: 0.3 21 0.44 0.24 0.75 0.016 0.014 — 0.0008 0.1040.0054 0.0010 Ti: 0.02, Mg: 0.0025, Sn: 0.04 22 0.41 0.25 0.75 0.0150.010 1.1 0.0005 0.192 0.0074 0.0019 Ti: 0.03, Mg: 0.0025 23 0.45 0.240.75 0.015 0.013 — 0.0009 0.119 0.0062 0.0019 Ti: 0.03, Mg: 0.0025, Cu:0.4 24 0.40 0..26 0.75 0.015 0.012 1.0 0.0005 0.198 0.0061 0.0015 Ti:0.02, Mg: 0.0025, Sn: 0.04 25 0.44 0.26 0.75 0.014 0.015 — 0.0008 0.1690.0060 0.0012 Ti: 0.03, Mg: 0.0025, Sn: 0.04, Cu: 0.3 26 0.42 0.24 0.750.014 0.013 1.0 0.0011 0.116 0.0074 0.0010 Ti: 0.03, Mg: 0.0025, Cu: 0.427 0.41 0.24 0.74 0.015 0.014 1.0 0.0004 0.198 0.0055 0.0012 Ti: 0.03,Sn: 0.04 28 0.46 0.25 0.75 0.015 0.010 — 0.0010 0.179 0.0061 0.0016 Ti:0.03, Sn: 0.04, Cu: 0.3 29 0.41 0.25 0.75 0.016 0.011 0.9 0.0008 0.1560.0062 0.0017 Ti: 0.04, Sn: 0.04, Cu: 0.3 30 0.41 0.26 0.75 0.014 0.0121.0 0.0009 0.137 0.0056 0.0012 Ti: 0.03, Cu: 0.3 31 0.45 0.24 0.75 0.0150.013 — 0.0013 0.109 0.0047 0.0010 Mg: 0.0025, Zn: 0.003 32 0.41 0.250.76 0.016 0.015 1.0 0.0011 0.104 0.0049 0.0013 Mg: 0.0019, Zn: 0.003 330.45 0.24 0.75 0.015 0.011 — 0.0013 0.109 0.0048 0.0014 Mg: 0.0022, Cu::0.3 34 0.40 0.25 0.75 0.016 0.015 1.0 0.0008 0.105 0.0046 0.0014 Mg:0.0020, Sn: 0.04 35 0.45 0.24 0.74 0.015 0.014 — 0.0009 0.110 0.00500.0018 Mg: 0.0025, Sn: 0.04, Cu: 0.1 36 0.42 0.25 0.75 0.014 0.012 1.00.0012 0.107 0.0047 0.0019 Mg: 0.0021, Sn: 0.02, Cu: 0.1 37 0.41 0.240.75 0.015 0.014 1.1 0.0005 0.104 0.0049 0.0019 Mg: 0.0029, Cu:: 0.1 380.42 0.25 0.76 0.015 0.011 1.0 0.0009 0.102 0.0049 0.0010 Sn: 0.04 390.44 0.24 0.76 0.015 0.012 — 0.0010 0.110 0.0046 0.0016 Sn: 0.03, Cu:0.1 40 0.41 0.25 0.75 0.015 0.011 1.0 0.0009 0.108 0.0046 0.0009 Sn:0.04, Cu: 0.1 41 0.41 0.25 0.75 0.015 0.011 0.9 0.0003 0.102 0.00460.0014 Cu: 0.2 42 0.46 0.25 0.76 0.015 0.011 — 0.0003 0.102 0.00550.0014 Nb: 0.01, Mg: 0.0026, Sn: 0.04, Cu: 0.3 Comparative 43 0.44 0.240.76 0.014 0.011 — 0.0006 0.025 0.0052 0.0018 Examples 44 0.45 0.25 0.760.015 0.014 — 0.0006 0.035 0.0051 0.0019 45 0.45 0.24 0.75 0.015 0.014 —0.0008 0.040 0.0053 0.0017 46 0.45 0.25 0.76 0.014 0.011 — 0.0010 0.0300.0034 0.0019 47 0.46 0.25 0.74 0.016 0.011 — 0.0008 0.003 0.0043 0.003448 0.44 0.24 0.75 0.014 0.009 — 0.0007 0.103 0.0058 0.0025

TABLE 8 Composition (mass %) No C Si Mn P S Cr Ca Ttl Al Ttl N Sol NOther Examples 52 0.45 0.26 0.75 0.016 0.025 — 0.0002 0.101 0.00520.0012 53 0.44 0.25 0.76 0.015 0.030 — 0.0000 0.250 0.0060 0.0013 540.45 0.25 0.74 0.015 0.042 — 0.0001 0.123 0.0048 0.0012 55 0.45 0.240.75 0.015 0.090 — 0.0002 0.106 0.0049 0.0013 56 0.45 0.24 0.75 0.0140.042 — 0.0001 0.102 0.0052 0.0011 Mg: 0.0020 57 0.45 0.24 0.74 0.0150.042 — 0.0001 0.190 0.0065 0.0016 Ti: 0.04 58 0.46 0.25 0.76 0.0160.047 — 0.0001 0.154 0.0075 0.0012 Nb: 0.02 59 0.45 0.25 0.74 0.0150.044 — 0.0001 0.129 0.0058 0.0017 W: 0.2 60 0.44 0.25 0.76 0.015 0.044— 0.0001 0.109 0.0055 0.0015 Ni: 0.2 61 0.45 0.26 0.74 0.016 0.041 —0.0001 0.148 0.0045 0.0015 Cu: 0.5 62 0.46 0.25 0.75 0.016 0.047 —0.0000 0.111 0.0052 0.0013 Sn: 0.03 63 0.46 0.25 0.75 0.015 0.051 —0.0001 0.188 0.0051 0.0012 Zn: 0.007 64 0.45 0.24 0.76 0.015 0.073 —0.0002 0.197 0.0053 0.0011 B: 0.002 65 0.44 0.25 0.75 0.015 0.092 —0.0002 0.109 0.0046 0.0010 Te: 0.002 66 0.45 0.25 0.74 0.015 0.062 —0.0000 0.200 0.0046 0.0011 Cr: 0.1 67 0.45 0.26 0.76 0.014 0.049 —0.0001 0.109 0.0070 0.0012 68 0.45 0.26 0.76 0.016 0.040 — 0.0000 0.1720.0072 0.0010 Ti: 0.03, Mg: 0.0025 69 0.45 0.25 0.75 0.014 0.040 —0.0001 0.110 0.0068 0.0010 Ti: 0.04, Zn: 0.004 70 0.41 0.25 0.75 0.0150.043 0.9 0.0000 0.125 0.0075 0.0009 Ti: 0.03 71 0.45 0.25 0.76 0.0150.043 — 0.0002 0.110 0.0069 0.0009 Ti: 0.03, Cu: 0.3 72 0.45 0.24 0.760.015 0.047 — 0.0000 0.125 0.0062 0.0018 Ti: 0.03, Mg: 0.0015, Sn: 0.0473 0.40 0.26 0.75 0.014 0.049 1.0 0.0001 0.142 0.0065 0.0017 Ti: 0.03,Mg: 0.0025 74 0.45 0.24 0.75 0.015 0.044 — 0.0001 0.149 0.0062 0.0017Ti: 0.03, Mg: 0.0025, Cu: 0.4 75 0.41 0.26 0.76 0.016 0.041 1.0 0.00010.129 0.0059 0.0019 Ti: 0.05, Mg: 0.0025, Sn: 0.04 76 0.44 0.24 0.760.015 0.043 — 0.0001 0.188 0.0061 0.0014 Ti: 0.03, Mg: 0.0025, Sn: 0.04,Cu: 0.3 77 0.40 0.26 0.75 0.016 0.046 0.9 0.0001 0.172 0.0064 0.0013 Ti:0.03, Mg: 0.0025, Cu: 0.4 78 0.41 0.25 0.75 0.016 0.042 1.0 0.0000 0.1110.0063 0.0013 Ti: 0.03, Sn: 0.04 79 0.46 0.25 0.76 0.015 0.047 — 0.00010.151 0.0067 0.0012 Ti: 0.03, Sn: 0.04, Cu: 0.3 80 0.40 0.26 0.76 0.0160.043 0.9 0.0001 0.120 0.0072 0.0017 Ti: 0.02, Sn: 0.04, Cu: 0.3 81 0.410.26 0.74 0.015 0.046 1.1 0.0001 0.144 0.0069 0.0010 Ti: 0.03, Cu: 0.382 0.46 0.24 0.76 0.014 0.040 — 0.0001 0.105 0.0051 0.0010 Mg: 0.0028,Zn: 0.003 83 0.41 0.24 0.76 0.014 0.047 0.9 0.0000 0.102 0.0052 0.0013Mg: 0.0019, Zn: 0.003 84 0.45 0.24 0.76 0.015 0.041 — 0.0001 0.1020.0069 0.0011 Mg: 0.0022, Cu:: 0.3 85 0.41 0.26 0.75 0.016 0.041 1.00.0000 0.109 0.0055 0.0012 Mg: 0.0020, Sn: 0.04 86 0.44 0.25 0.76 0.0160.047 — 0.0001 0.103 0.0062 0.0010 Mg: 0.0025, Sn: 0.04, Cu: 0.1 87 0.420.26 0.75 0.015 0.042 1.0 0.0001 0.101 0.0057 0.0011 Mg: 0.0017, Sn:0.04, Cu: 0.1 88 0.41 0.25 0.75 0.015 0.046 1.1 0.0001 0.106 0.00670.0013 Mg: 0.0025, Cu:: 0.1 89 0.41 0.25 0.74 0.014 0.046 1.0 0.00000.109 0.0059 0.0016 Sn: 0.02 90 0.45 0.26 0.75 0.015 0.042 — 0.00010.100 0.0066 0.0013 Sn: 0.04, Cu: 0.1 91 0.41 0.24 0.74 0.015 0.046 1.10.0001 0.105 0.0065 0.0012 Sn: 0.04, Cu: 0.1 92 0.41 0.26 0.75 0.0150.040 1.1 0.0000 0.109 0.0058 0.0014 Cu: 0.1 93 0.44 0.24 0.75 0.0150.057 — 0.0001 0.101 0.0051 0.0017 Nb: 0.01, Mg: 0.0025, Sn: 0.04, Cu:0.3 Comparative 94 0.45 0.25 0.74 0.014 0.026 — 0.0001 0.025 0.00510.0017 Examples 95 0.45 0.24 0.75 0.014 0.043 — 0.0001 0.024 0.00520.0018 96 0.46 0.24 0.75 0.016 0.046 — 0.0002 0.032 0.0051 0.0019 970.46 0.24 0.76 0.015 0.046 — 0.0002 0.104 0.0078 0.0034 98 0.45 0.250.74 0.016 0.043 — 0.0001 0.103 0.0058 0.0025 99 0.44 0.26 0.76 0.0160.051 — 0.0000 0.243 0.0057 0.0026 100 0.45 0.24 0.75 0.014 0.073 —0.0001 0.111 0.0067 0.0031 101 0.46 0.25 0.75 0.014 0.099 — 0.0001 0.1420.0077 0.0035

Machinability Test

Machinability testing was conducted with respect to Example andComparative Example steels that had been elongation-forged under heatingat 1250° C. by first subjecting them to heat treatment consisting ofnormalization under temperature condition of 850° C. for 1 hr, 0.5 hr inthe case of Comparative Examples No. 48, No. 49 and No. 97 to No. 101,followed by air-cooling. A machinability evaluation test piece was thencut from each heat-treated steel and the machinabilities of the Exampleand Comparative Example steels were evaluated by conducting drill boringtesting under the cutting conditions shown in Table 9 and tolongitudinal turning testing under the conditions shown in Table 10. Themaximum cutting speed VL1000 enabling cutting up to a cumulative holedepth of 1000 mm was used as the evaluation index in the drill boringtest, and the maximum width VB_max of wear of the relief flank after 10min was used as the evaluation index in the longitudinal turning test.

TABLE 9 Cutting Speed 10-120 m/min conditions Feed 0.25 mm/rev CuttingWater-soluble fluid cutting oil Drill Drill 3 mm diameter (φ) NACHIOrdinary drill Overhang 45 mm Other Hole depth 9 mm Tool life Untilbreakage

TABLE 10 Cutting Cutting 250 m/min conditions speed Feed 0.3 mm/rev ModeDry cutting Tool Holder PTGNR2525M16 Shape TNMG160408N-UZ MaterialAC2000

Charpy Impact Test

FIG. 1 is a diagram showing a region from which a Charpy impact testpiece was cut. In the Charpy impact test, first, as shown in FIG. 1, acylinder 2 measuring 25 mm in diameter was cut from a steel 1heat-treated by the same method and under the same conditions as theaforesaid machinability test piece so that its axis was normal to theelongation-forging direction of the steel 1. Next, the cylinder 2 washeld under temperature condition of 850° C. for 1 hr, 0.5 hr in the caseof Comparative Examples No. 48, No. 49 and No. 97 to No. 101, thenoil-quenched by cooling to 60° C., and further subjected to temperingwith water cooling in which it was held under temperature condition of550° C. for 30 min. Next, the cylinder 2 was machined to fabricate aCharpy test piece 3 in conformance with JIS Z 2202, which was subjectedto a Charpy impact test at room temperature in accordance with themethod prescribed by JIS Z 2242. Absorbed energy per unit area (J/cm²)was adopted as the evaluation index.

Tensile Test

A cylinder 2 oil-quenched and tempered by the same methods and under thesame conditions as in the aforesaid Charpy impact test was processedinto a tensile test piece measuring 8 mm in parallel section diameterand 30 mm in parallel section length, and then tensile tested at roomtemperature in accordance with the method prescribed by JIS Z 2241.Yield ratio (=(0.2% proof stress YP)/(tensile strength TS) was adoptedas the evaluation index.

The results of the foregoing tests are shown in Tables 11 and 12.

TABLE 11 Impact VL1000 VB_max value No. (m/min) (μm) (J/cm²⁾ YP/TSExamples 1 70 121 39 0.87 2 65 121 40 0.82 3 65 123 41 0.84 4 65 125 420.80 5 70 115 39 0.83 6 65 121 42 0.83 7 65 123 41 0.86 8 65 120 40 0.859 65 116 42 0.85 10 65 122 43 0.86 11 70 123 44 0.85 12 70 120 38 0.8313 70 119 39 0.84 14 70 120 40 0.85 15 55 132 37 0.89 16 65 124 40 0.8617 65 125 40 0.82 18 70 124 39 0.85 19 55 131 39 0.84 20 65 126 38 0.8221 70 118 39 0.83 22 55 133 39 0.84 23 70 128 38 0.83 24 60 130 39 0.8525 70 119 39 0.83 26 55 131 40 0.83 27 65 132 40 0.83 28 70 121 40 0.8429 60 131 39 0.86 30 55 131 38 0.85 31 70 120 41 0.83 32 55 133 37 0.8633 70 125 40 0.87 34 60 134 39 0.86 35 70 120 39 0.87 36 60 133 41 0.8737 55 131 41 0.84 38 60 130 38 0.86 39 70 119 39 0.86 40 60 131 38 0.8441 55 132 37 0.84 42 65 124 41 0.83 Comparative 43 45 122 41 0.66Examples 44 45 116 40 0.67 45 45 117 41 0.67 46 50 110 42 0.67 47 35 15622 0.68 48 50 149 30 0.87 49 50 140 29 0.85

TABLE 12 Impact VL1000 VB_max value No. (m/min) (μm) (J/cm²⁾ YP/TSExamples 52 85 121 25 0.85 53 85 123 24 0.85 54 95 121 23 0.82 55 105112 21 0.85 56 95 120 22 0.85 57 95 123 22 0.85 58 90 123 22 0.85 59 95121 22 0.83 60 90 124 22 0.83 61 95 120 22 0.87 62 95 115 22 0.83 63 95125 22 0.85 64 100 117 21 0.84 65 105 113 21 0.84 66 95 121 20 0.86 6780 131 20 0.83 68 95 122 25 0.87 69 100 120 24 0.84 70 80 131 20 0.84 7195 122 23 0.85 72 100 118 26 0.83 73 80 130 21 0.84 74 95 122 25 0.85 7585 128 20 0.85 76 100 119 25 0.86 77 80 132 22 0.83 78 85 128 20 0.82 79100 120 24 0.82 80 85 126 21 0.84 81 80 133 21 0.83 82 105 120 23 0.8483 80 130 21 0.83 84 95 124 24 0.86 85 85 129 22 0.87 86 95 117 23 0.8787 85 128 21 0.84 88 80 129 21 0.86 89 85 126 20 0.83 90 95 119 21 0.8691 85 125 20 0.87 92 80 133 20 0.83 93 100 112 22 0.86 Comparative 94 60180 22 0.68 Examples 95 65 179 20 0.69 96 65 174 19 0.68 97 70 157 180.84 98 75 149 18 0.82 99 70 143 18 0.86 100 75 152 15 0.79 101 75 16312 0.86

The steels No. 1 in Tables 7 and 11 are embodiments of claim 1 and thesteels No. 2 to No. 42 in the same tables are embodiments of claim 2.The steels No. 52 to No. 93 in Table 8 and Table 12 are embodiments ofclaim 1. The comparative steels No. 43 to No. 49 satisfy the S contentand Ca content requirements of claim 2, and the comparative steels No.94 to No. 101 satisfy the S content and Ca content requirements of claim1.

As shown in Tables 11 and 12, the steels of Examples No 1 to No. 42 andNo. 52 to No. 93 exhibited good values for all of the evaluationindexes, namely VL1000, VB_max, impact value (absorbed energy), andYP/TS (yield ratio), but the steels of the Comparative Examples wereeach inferior to the Example steels in at least one of the properties.Specifically, the steels of Comparative Examples No. 43 to No. 46 hadtotal Al contents below the range of the present invention and weretherefore inferior to the Example steels in machinability (VL1000) andyield ratio (YP/TS). Moreover, the steel of Comparative Example No. 47had a total Al content below the range of the present invention, so thatits solute N content was above the range of the present invention, andthe steel was therefore inferior to the steels of the Examples inmachinability (VL1000, VB_max), impact value, and yield ratio (YP/TS).

The steels of Comparative Examples No. 48 and No. 49 were maintained at850° C., the temperature at which AlN readily precipitates, for ashorter holding time than the steels of the Examples, so that theirsolute N contents were above the range of the present invention, and thesteels were therefore inferior to the steels of the Examples inmachinability (VL1000, VB_max) and impact value. Moreover, the steels ofComparative Examples No. 94 to No. 96 had a total Al content below therange of the present invention and were therefore inferior to the steelsof the Examples in machinability (VL1000, VB_max) and yield ratio(YP/TS). Further, the steels of Comparative Examples No. 97 to No. 101were maintained at 850° C., the temperature at which AlN readilyprecipitates, for a shorter holding time than the steels of theExamples, so that their solute N contents were above the range of thepresent invention, and the steels were therefore inferior to the steelsof the Examples in machinability (VL1000, VB_max) and impact value.

INDUSTRIAL APPLICABILITY

The present invention provides a machine structural steel that has goodmachinability over a broad range of machining speeds and also has highimpact properties and high yield ratio.

1. A machine structural steel excellent in machinability and strength properties comprising, in mass %: C, 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, total Al: greater than 0.05% and not greater than 0.3%, Sb: less than 0.0150% (including 0%), and total N, 0.0035 to 0.020%, solute N being limited to 0.0020% or less, and a balance of Fe and unavoidable impurities.
 2. A machine structural steel excellent in machinability and strength properties according to claim 1, further comprising, in mass %, one or more of Ca: 0.0003 to 0.0015%, Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, V: 0.01 to 1.0%, Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, REMs: 0.0001 to 0.015%, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5%, Pb: 0.005 to 0.5%, Cr: 0.01 to 2.0%, Mo: 0.01 to 1.0%, Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0%. 